Conventional types of digital data storage systems include compact disc read only memory devices (CD players and CD-ROM devices) and digital audio tape recorder/playback devices (DAT devices).
FIG. 1 depicts a functional block diagram of a conventional DAT device. As can be seen in FIG. 1, the conventional DAT device includes a recording section comprised of an analog-to-digital (A/D) converter 1, a digital signal processor (DSP) 2, a digital memory 3, a recording amplifier 4, and a rotary head (labelled "HEAD"). In operation, an analog audio signal to be recorded is inputted to the A/D converter 1, which functions to convert the analog audio signal into a digital signal. The DSP 2 then compresses and encodes the digital signal in accordance with an EFM (eight-to-fourteen bit) encoding scheme, which incorporates a Cross-Interleaved Reed-Solomon Error Correction Code (CIRC), to thereby interleave the resultant pulse-coded modulation (PCM) digital data blocks in a non-sequential manner and provide an Error Correction Code (ECC) sub-block for each of the PCM digital data blocks. The thusly EFM-encoded digital data is then stored in digital memory 3, to facilitate time-base compression of the data. The EFM-encoded digital data is then read out of the digital memory on a first-in, first-out (FIFO) basis, and applied to the recording amplifier 4, which amplifies the EFM-encoded digital signal (which constitutes a representation of the original input analog audio signal). The thusly amplified EFM-encoded digital audio signal is then recorded on a digital audio tape (which is driven by the "CAPSTAN") by the rotary head ("HEAD"). The recording amplifier 4 is also responsive to an automatic track-finding (ATF) signal supplied thereto by an ATF signal generator 5, to thereby ensure that the ATF signal is recorded on an ATF region of the digital audio tape.
With continuing reference to FIG. 1, it can be seen that the conventional DAT device also includes a playback section comprised of a playback amplifier/waveform equalizer/signal detector 6, the DSP 2, the digital memory 3, a digital-to-analog (D/A) converter 7, a servo system 8, and a system controller 9. In operation, the rotary head functions to read the recorded EFM-encoded digital audio signal from the digital audio tape. The read-out digital audio signal is then amplified and equalized by the playback amplifier/waveform equalizer/signal detector 6. The output of the detector 6 is then fed to the DSP 2, which expands (de-compresses), demodulates, decodes, de-interleaves and performs error correction on the amplified and equalized digital audio signal. The D/A converter 7 then converts the decoded, error-corrected digital audio signal output by the DSP 2 into an analog audio signal, which is a close reproduction of the original analog audio signal represented by the recorded digital audio signal read from the digital audio tape. The detector 6 also functions to extract the ATF signal from the digital signal read from the digital audio tape. This ATF signal is then fed to the servo system 8, which uses this ATF signal as a tracking servo control signal to maintain the rotary head in accurate relationship to the proper information track on the digital audio tape. Further, the system controller 9 functions, in response to a control signal supplied thereto by an envelope detector 13, to control the rate of travel of the digital audio tape. The system controller 9 also processes sub-code data extracted from the read-out digital audio signal to facilitate user interface with a keypad 10 and display 11. The clock generator 12 generates the timing signals for synchronizing the operation of the various components of the overall system. The servo system 8 also includes a drum servo portion (not shown) and a capstan servo portion (not shown). The drum servo portion is comprised of a speed control circuit, a phase control circuit, and a bias control circuit, which cooperatively function to rotate the rotary head at a precisely accurate rotational speed and to precisely maintain a proper phase relationship between the read-out digital signal and a reference phase. The capstan servo portion is comprised of a speed control circuit, an ATF circuit, and a phase control circuit, which cooperatively function to more precisely control the rate of travel of the digital audio tape at a constant speed during both recording and playback operations.
The above-described conventional DAT device suffers from the following drawbacks and shortcomings. More particularly, although the data transfer rate (i.e., recording and playback speed) of a DAT device is faster than that of a CD-ROM device, it is still slow. Further, DAT devices utilize moving mechanical parts, such as a rotary head and tape drive, which are subject to degradation over time due to wear and tear and exposure to varying environmental conditions, thereby limiting the reliability and useful life thereof, and increasing the maintenance cost thereof. Moreover, because DAT devices use mechanical parts and an RF amplifier, among other things, the integrity of the data processed thereby is compromised. Additionally, the use of mechanical parts which must be precisely servo-controlled entails the use of complex electronic servo control componentry which imposes constraints on the cost, size, and complexity of the device. Furthermore, the data packing density (i.e., memory capacity) of the digital audio tape is limited by several factors, including separation and gap losses, the size of the magnetic particles whose orientation represents the recorded data, and the rotational frequency of the rotary head.
The conventional CD-ROM devices suffer from drawbacks and shortcomings similar to those described above in connection with DAT devices. Although the optical pickup head used in CD-ROM devices is not very susceptible to wear and tear, because it is only movable in fine increments for focus and tracking purposes, it is difficult to manufacture and servo control, thereby imposing constraints on the cost, size, complexity, and reliability of the device. Moreover, an optical carriage/sled is required to transport the optical pickup head radially across the surface of the compact disc (CD), and this sled, since it is a moving, mechanical component, is susceptible to malfunction and wear and tear, thereby imposing further constraints on the cost, size, complexity, and reliability of the device. Further, the CD is rotated by a motor-driven spindle/turntable mechanism, which not only imposes yet further constraints on the cost, size, complexity, and reliability of the CD-ROM device, but also imposes limitations on the playback speed of the device. Not only is the useful life of the CD-ROM device limited by virtue of the abovedescribed mechanical componentry, it is further limited by virtue of its use of a diode laser to generate the optical read beam which is used to detect the data recorded on the CD, since a diode laser has a finite life specified by the manufacturer thereof. Furthermore, the data packing density (i.e., memory capacity) of the CD is limited by several factors, including the wavelength of the laser beam generated by the diode laser, the response rate of the photodetectors used to detect the intensity of the modulated light reflected from the surface of the CD during playback, the linear speed of the optical carriage/sled, and the rotational velocity of the spindle/turntable mechanism. Yet further, CD-ROM devices are susceptible to interruption or complete shutdown during playback due to such factors as vibration, shock, and jerky movement of the device, e.g., such as is commonly experienced in CD players installed in automobiles or transportable CD players.
Based upon the above, there presently exists a need in the art for a digital data storage system which overcomes the above-described drawbacks and shortcomings of conventional digital data storage systems such as DAT and CD-ROM devices. The present invention fulfills this need.